Acoustic coupling device



March 13, 1951 R. H. POSTAL 2,544,765

ACOUSTIC COUPLING DEVICE Filed Aug. 25, 1948 \NVENTOR Poker!" H- P0515221 ATTORNEY Patented Mar. 13, 1951 ACOUSTIC COUPLING DEVICE Robert H. Postal, New York, N. Y., assignor to Thomas A. Edison, Incorporated, West Orange, N. J., a corporation of New Jersey Application August 23, 1948, Serial No. 45,772

V 1 Claim. 1 This invention relates to novel and improved acoustic coupling and/or modifying devices, and

particularly to a unitary such device for coupling a sound-transmitting tube to a sound box of a phonographic translating unit and for controlling the tonal balance of the sound.

The invention is herein particularly shown and described, by way of illustration, in connection with a commercialform of reproducingtype phonographic translating device such as is used for reproducing recorded dictation, but it will be understood that the invention has utility also with other reproducing devices as well as with phonographic recording devices.

,An object of my invention is to provide a device for coupling a sound-transmitting tube to a sound-box, which will efliciently transmit the sound from one to the other without reflection losses or phase interference.

It is another object to provide such a device which will transmit efficiently the sound between the sound-box and tube while maintaining a substantial phase equalization of the higher-audio frequencies.

It is another object to provide such a device which is adapted. for coupling efficiently a soundbox to small sound-transmitting tubes having an internal diameter of the order of .1" or less.

A more particular object is to provide such a device for efficiently transmitting sound with phase equalization from a diaphragm to a sound receiver through a sound-transmitting tube.

Another object is to provide a unitary device" of the character described which is adapted to serve also as a sound modifier.

Another object is to provide such a sound coupler and modifier which is adapted particularly for-use in the frequency range of the speech sounds.

Still another object is to provide such a device which enables eflicient transmission of speech sounds between a sound-box of a phonographic reproducing unit and a sound receiver through a-- sound-transmitting tube, and which additionally provides a tone control adapted to enable the recordation and reproduction of speech sounds with maximum intelligibility.

These and other objects and features of my-- invention will be apparent from the following description and the appended claims.

In the description of my invention reference is had to the accompanying drawings, of which:

Figure 2 is a top plan view of the device of Fi ure 1, as seen from the line 2-2 of that figure;

Figure 3 is a front elevational view of the device of Figure 1, showing a section of a soundtransmitting tube and receiver to reduced scale;

Figure 4 is a fractional sectional view taken on the line 44 of Figure 1;

Figures 5 and 6 are side elevational and bottom plan views respectively of an internal part of the coupling device of my invention;

Figure 7 is a fractional axially sectional view of the sound-coupling device of my invention shown to enlarged scale; and

Figure 8 is an electrical analogue of the acoustic system of the present translating unit with my invention.

In machines for reproducing recorded dictation, as from a record R typically of cylindrical form, there is a carriage I0 fractionally shown which is moved progressively along the record as the record is rotated. This carriage has a shouldered mounting hole H for receiving a reproducing-type phonographic translating unit l2 herein sometimes referred to simply as a reproducer. Typically, reproducers for recorded dictation are of the acoustic type, as shown, wherein the reproduced sounds are produced mechanically directly by the reproducer-stylus as it tracks a modulated groove of the record. My invention is particularly concerned with improved means for conducting the sound from such a reproducer to the ear through a transmitting tube, but in a broader sense the invention contemplates such improved means for conducting sound to and/or from any sound chamber having a vibratile diaphragm for producing and/or receiving sounds.

The reproducer [2 has an annular frame l3 open at the top and provided with an internal annular shoulder l4. Mounted on this shoulder is a circular vibratile diaphragm l5 having a gasket ring IE on its rim portion to seal it airtight to the frame. Mounted on the upper face of this gasket ring is a closure member I! (hereinthe sound chamber [8 is commonly-referred to as a sound-box.

Depending from the forward portion of the frame [3 is a lug l9 carrying a transverse pin 29 at its lower end. Pivoted on this in is a bracket 2 I which is disposed below the reproducer.

Figure 1 is a sideview of a phonographic reproducer incorporating the sound coupler and modifier of my invention, with parts being shown in elevation and other parts in section on a plane through the central axis of the reproducer;

This bracket has two vertically spaced and rearwardly extending ears 22 and 23 of which the upper one is apertured to form a bearing. Threaded through the lower ear'23 is a screw 24 having a conical bearing at its inner end confronting the other bearing just mentioned. Engaging these bearings are cone pivots of a biasing weight 25 which extends across the record R. This biasing weight thus has a transverse freedom of movement on the cone pivots just mentioned and has an up and down freedom of movement on the pin 20. At the rearward end of this weight is a pin 26 which lies within a. loop 21 depending from the rearward portion of the frame. This pin andloop serve to limit the range of transverse and up and down movement of the weight.

Depending from the biasing weight 25 is an apertured lug 28 to which a stylus lever 29 is pivoted. Oneend of this stylus lever is coupled by a link 30 to the central portion of the diaphragm i and the other end thereof has mounted thereon a reproducing stylus it for engaging the record Hi. When the biasing weight 25 is in a free released condition, as shown, the stylus is urged sound receiver at its outer end adapted to be fitted on or into the ears of a listener, a common form. 32a. of such receiver being shown to reduced scale in Figure 3..

The lower end of the frame. I3 is preferably closed by" an. integral'wall 6c. of the frame l3, which. wall is provided with a long clearance opening 6-! at its central portion through which the 30 passes. This wall forms a relatively large air chamber 6.2- below the diaphragm l5, which has a compliance resonant with the acoustic in ertance of the opening 8|- at the lower end of the speech frequency spectrum. Such an acoustic chamber and opening constitute eifective'ly a lowpass: filter which will attenuate the radiation of all sounds above its cutoff frequency from the bottom side of the diaphragm to the outside atmosphere. This means for attenuating sounds fromv the; bottom of the diaphragm is not herein claimed since the same is the subject matter of the pending application S. N. 699,675 of John Hornbostel, filed September 27, 1946, now Patent Number 2,541,163, issued February 13, 1951, and having common ownership with the present application.

The. device, of my invention for coupling the sound chamber 8 to the transmitting tube 32 com-prises three arcuateopenings 33 in the closure member H (Figure 6 These openings form effectively a single annular opening and are in three. sections only so that the narrow intervening strips. 34' therebetween will provide adequate support. for the central portion 35 of the closure member. This central portion 35 constitutes. the base-of. an upstanding cone 3-5. The axis of: this cone: andof the openings 35 is positioned. at the center of the diaphragm I5. Seated on the exterior orrim portion of the closure member IT is; a flanged base 3-8 of a coupling member 39 which is,- held in, place by a clamping ring 40 thread-ed. internally into. the. frame 13.. This coupling member has a central flared opening 4| surrounding the cone 35- in spaced relation thereto so as to form therewith an annular sound duct 42 leading from the sound chamber I8 and having a decreasing mean diameter along the axis of the cone. Beyond the cone the annular duct 42 merges into a cylindrical one 4211 which leads through a. nipple 42b to the sound transmitting tube 32. However, in the cylindrical duct 42a there is preferably interposed a valve 43 that is journalled in the coupling member and which has a knurled head 44 operableby hand to serve as a volume control. As shown in Figure 2, the head may have a stop pin thereon adapted to abut against the top face of the coupling member 39 to define its limit positions, and as shown in Figure 3, the coupling member 38 may have a dial face 46 back of the head 44 and provided with suitable markings, as O and C, to indicate in connection with anarrow on the head when the valve is. inopen and closed positions. 7

The present coupling device is better understood by referenceto the electrical analogue of the sound-box of the present reproducer, shown V and the resistance R refers to the efiective. re-

sistance of this sound duct when coupled to the ear. Preferably, the air chamber I3 is made very small, say about .015 high, so that its compliance C2 will be very small compared to the compliance C1. of the diaphragm. Thus, the capacity C1 may bev ignored, and the network of Figure 8 may be considered as a simple low-pass filter. This filter must pass all the speech frequencies and therefore its cut-oh frequency should be at the upper end of the speech frequency spectrum. By way of illustration, the diaphragm and air chamber may have an effective diameter of 5.8- cmand the diaphragm a mass of. .17 g. For these values, the cut-off frequency of the filter occurs at 7500 C. P. S. Also, the characteristic impedance of this filter phragm just noted is of. the order of 3 sq. cms.

The effective area of the duct 42-at its junction with the sound chamber I8 is to be determined, andis referred to as AT. The mechanical resistance R of the sound duct 42 is of equal the characteristic impedance of the V filter transformed by the-impedance ratio of the transformer-4. e-., R is toequal for the typical values assumed. On the other hand, as an approximate and simplifying assumption, the resistance R of the duct 42 may be computed, when the duct is terminated into the ear, on the basis that the duct is one of infinite length. On this assumption, the resistance R would be POVAT where P0 is the density of air (1.2 1'0 gms./c. c.) and v is the velocity of sound in air (38.3)( cms./sec.). Upon equating these two values of the resistance R, it is found that the cross-sectional area A'r of the sound duct 42 should be .09 sq. cm. in order that the resistance thereof will match approximately the characteristic impedance of the sound'box. When this impedance match is obtained, reflection losses are avoided or at least mitigated with the resultant greater efiiciency in sound transmission from the soundbox to the tube 32. 7

It is found that even though the sound outlet duct 42 has a resistance equal to the characteristic resistance of the soundbox, the pressure waves from different parts of the diaphragm tend to arrive at the transmitting tube in out-cfphase relation, particularly in the higher-frequency speech spectrum, to cause the response characteristic through the higher portion of the audio spectrum to be badly broken up. This causes not only distortion, as evidenced by the reproduced speech having a raspy character, but causes also a marked reduction in the average level of the higher-frequency speech sounds. In order to mitigate these out-ofphase effects, (1) the sound duct 42 leading from the sound chamber I8 is made annular as described; (2) this sound duct is preferably positioned symmetrically to the diaphragm-i. e., with its axis at the center of the latter; and (3) the diameter of the sound duct 42 at its junction with the sound chamber is preferably made substantially equal to one-half the diameter of the diaphragm sothat each element of the sound duct is substantially midway between the center of the diaphragm and its periphery. By leading the sound out of the soundbox through such an annular sound duct, the pressure waves produced by each element of the diaphragm arrive at the transmitting tube 32 with nearly equal phase relationship to enable more even and a higher level of response of the higher frequency sounds with a resultant greater conveyance of speech intelligibility.

A more even and efficient conveyance of the higher-frequency speech sounds from the soundbox to the transmitting tube 32 permits the transmitting tube to be smaller in diameter for any given level of speech transmission, it being understood that the smaller the diameter of the transmitting tube the poorer is its transmission of the higher-frequency sounds. It is particularly beneficial to be able to use a small and light transmitting tube since then the transcriber is afforded a greater degree of freedom of movement. For instance, with the benefit of the present invention, speech sounds can be received as efiiciently from the soundbox with the use of a transmitting tube of only .1 inch internal diameter as has been heretofore possible with the use of a transmitting tube having an internal diameter of .25 inch.

When the cross-sectional area of the sound duct s2 at its junction with the sound chamber i8 is equal to that of the transmitting tube 32, the area of the annular duct 42 is maintained constant along the length thereof. However, when the cross-sectional area of the transmitting tube is different from that of the sound duct at its junction with the soundbox, the area of the sound duct is varied preferably exponentially along the length of the duct so that it will have an area at its junction with the transmitting tube equal to that of the latter. In Figure '7, there is an enlarged view of a section of the annular duct 42 when the cross-sectional area is varied exponentially from a value of .09 sq. cm. at the sound chamber to a value of .0078 sq. cm. at the tube 32.

In order that the relative levels of the lowerand higher-frequency portions of the reproduced sounds may be adjusted for best intelligibility a .tone control 413 is provided as a part of the coupling device 39 of my invention. This tone control comprises a series of openings 49 in the closure member i1 (Figure 6) and a long arcuate opening 50 in the base of the coupling member 39, which is positioned to expose the series of openings 49. However, interposed between the coupling and closure members is an annular shutter 51 that fits into an annular recess 52 in the bottom of the coupling member. This shutter has an arcuate opening 53 which can be moved into registration with one or more of the openings 49 by manually turning the shutter as by means of a fingerpiece 5m projecting upwardly through the opening 50 from the shutter. To prevent the closure member I1 from being then also turned through its frictional contact with the shutter, the closure member has a locking tab l'la engaging a hole 13a in the frame l3 (Figures 1, 2 and 5) When the fingerpiece is turned to the right the shutter closes the openings 49 and when it is moved to the left it successively opens these openings 49 (Figures 3 and 4). These openings provide an outlet from the sound chamber l8 to the outside atmosphere. Since such openings have a small inertance to transmission therethrough of the low-frequency sounds and a high inertance to transmission therethrough of highfrequency sounds they serve to attenuate lower frequencies without substantially affecting the level of response of the higher frequencies. Because of the natural tendency in acoustic-type phonographic systems for the higher-frequency sounds to be attenuated relative to the lower-frequency sounds, the lower-frequency sounds sometimes have such a great relative level as to mask the higher-frequency sounds and destroy thus in great measure the intelligibility of the reproduced speech. When such a condition occurs one or more of the openings 49 may be opened, as needed, to attenuate the lower-frequency sounds sufficiently to balance the speech frequencies for best intelligibility. A feature of this tone control is in its adaptation to the sound coupling means of my invention and in its provision therewith as a unitary device.

The particular embodiment of my invention herein shown and described is intended to be illustrative and not limitative of my invention, since the same is subject to changes and modifications without departure from the scope of my invention, which I endeavor to express according to the following claim.

I claim:

In an acoustical-type receiving device adapted to be coupled to the sound box of a phonographic reproducer, said sound box including a vibratile diaphragm: the combination of a coupling member mounted on said sound box and forming a sound chamber between it and said diaphragm, said coupling member having a duct with a central plug therein to provide an arcuate opening leading from said sound chamber the diameter 7 7' ofi opening at the sound chamber" substantially oneshalfi the: diameter of the; dianhragm, said plugbeing substantially conical in shape and; said arcuatel opening. merging into a cylindrical opening beyond said conical plug at the outer end of said coupling member, a sound tube connected to the outer end. of said coupling member in communication with said cylindrical opening, and an acoustic-type sound receiver connected to the outer end of said sound tube and adapted" to be connected to the ear of a listener, wherein the ratio of" the area of said diaphragm and the cross sectional area of'said arcua te' opening at" its junction with said" sound chamber is equal approximately to the ratio of theimpedance of the sound chamber at said arcuate opening and the impedance of said sound tube computed on the basis of the tube having an infinite length, and wherein the" cross sectional I area of; said. arcuate opening; along the: length thereof? froma said sound: chamber to" said: sound tube. is varied: substantially exponentially..

ROBERT. POSTAL.

REFERENCES CITED- The following references are" of record" in the file of" this patent:

10 UNITED STATES PATENTS Number Name Date 1,052,656 Edison Feb. I1, 1913 1,379,407 Hensley May 24, 1921 1,456,538 Crandall" May 29, 1923 15 1,4393211- Head Apr. I, 1924 1,345,768v Stokes Feb. 16; 1932 1,907,723 Bostwick May 9; 1933 2,037,185 Thures- Apr. 14', 1936 2,102,668 Ballantine Dec. 21, 1937 20 

